The sound-isolating earphone is an ear plug structure as a whole comprising a sound emitting portion with its rear face closed, and an ear pad having a sound exit at the distal end of a portion to be inserted into the external auditory canal formed of soft plastic, rubber or the like having elasticity which is in close contact with the inner face of the external auditory canal without a gap. Since the sound-isolating earphone can be attached by inserting the ear pad into the external auditory canal, the sound-isolating earphone can be reliably attached to the entrance of the external auditory canal. Also, the ear pad is made of a material having flexibility so that the ear pad can be elastically deformed easily in accordance with the shape of the external auditory canal and can provide favorable wearing feeling.
As a result, the sound-isolating earphone which is used by being inserted into the entrance of the external auditory canal has favorable sealing performances, provides high sound isolation, and reduces hearing of external noise, and thus, high sound pressure sensitivity can be obtained and feeble sound can be heard even in a very noisy place. Also, since it can be used by being inserted into the entrance of the external auditory canal, it has an advantage that reduction in size and weight is easy.
In recent years, with spread of portable music players, development of a sound-isolating earphone capable of outputting sound with a good sound quality is in increasing demand.
However, since a prior-art sound-isolating earphone has a structure to seal the external auditory canal, the state of resonance in the external auditory canal changes between before and after the attachment of the earphone, and resonance frequency is displaced and causes a significant defect in the frequency characteristic of the earphone.
Referring to FIG. 1, this point will be described below. FIG. 1 is a schematic diagram of an external auditory canal. When a human being listens to sound, vibration of air generated outside passes an external auditory canal entrance 7 and an external auditory canal 8 and then, reaches an eardrum 9 and vibrates the eardrum 9.
At this time, the external auditory canal 8 is, as illustrated in FIG. 1(a), in a state in which one end is closed by the eardrum 9 and the external auditory canal entrance 7, which is the other end, is opened to the atmosphere. That is, it is in a state of a pipe with one end closed and the other end open (hereinafter referred to as one-end closed pipe). Therefore, one-end closed pipe resonance using the external auditory canal 8 as a resonance box occurs. If the one-end closed pipe resonance occurs, standing waves occur and such resonance occurs that the vibration of air at the closed end of the closed pipe becomes the minimum (pressure variation is the maximum), and the vibration of air at the open end of the closed pipe becomes the maximum (the pressure variation is the minimum).
FIG. 1(b1) and FIG. 1(b2) schematically illustrate the state in which the one-end closed pipe resonance occurs. A solid line indicates a resonance box of the one-end closed pipe, while a broken line indicates amplitude of air vibration.
The frequency characteristics when a sound wave passes through the external auditory canal including the resonance state are found as follows: An expression p1 of a sound wave having a wavelength λ travelling at a speed V from the external auditory canal entrance 7 to the eardrum 9 (this is referred to as a +x direction) at time t can be expressed as follows. Here, reference character A is an arbitrary value:p1(x,t)=A sin {2π(x−Vt)/λ}.Similarly, a sound wave p2 reflected by the ear drum 9 and travelling at the speed V to the external auditory canal entrance 7 (this is referred to as a −x direction) can be expressed as follows:p2(x,t)=A sin {2π(x+Vt)/λ}.
Since an advancing wave and a sound wave reflected by a closed bottom and returned coexist in the one-end closed pipe, a sound wave P obtained by synthesizing the both can be expressed as follows:
                              P          ⁡                      (                          x              ,              t                        )                          =                ⁢                              p            ⁢                                                  ⁢            1            ⁢                          (                              x                ,                t                            )                                +                      p            ⁢                                                  ⁢            2            ⁢                          (                              x                ,                t                            )                                                              =                ⁢                              Asin            ⁢                          {                              2                ⁢                                                      π                    ⁡                                          (                                              x                        -                        Vt                                            )                                                        /                  λ                                            }                                +                      Asin            ⁢                          {                              2                ⁢                                                      π                    ⁡                                          (                                              x                        +                        Vt                                            )                                                        /                  λ                                            }                                                              =                ⁢                              Asin            ⁡                          (                              2                ⁢                                  π                  /                  λ                                            )                                ×                                    sin              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      Vt                    /                    λ                                                  )                                      .                              
When this is rewritten using a frequency f with the relationship of λ=V/f,P(x,t)=A sin(2πxf/V)×sin(2λtf)  (Formula 1)is obtained.
The first half of the formula of the synthesized sound wave P shows the amplitude at a position x regardless of time, while the second half shows a temporal fluctuation portion, which indicates a standing wave, not a traveling wave. A point where the amplitude is the maximum all the time irrespective of the time t is found as follows:sin 2πx/λ=1.Therefore,2πx/λ=±(2n−1)π/2.Considering only the positive part of the x-coordinate, it is x=(2n−1)λ/4, where n is a positive integer.
Since the resonance state occurs only when the distance between the points where the amplitude is the maximum all the time is the same as a length L of the resonance box, substituting x=L in the above formula, and obtainL=(2n−1)λ/4.Here, since λ=V/fL=(2n−1)V/4f, ∴f=(2n−1)/V/4L  (Formula 2)is true.
As described above, the resonance of the one-end closed pipe occurs when the length of the resonance box is (2n−1) times as long as one-fourth wavelength. Here, n is a positive integer. FIG. 1(b1) shows the state of primary resonance (n=1), while FIG. 1(b2) shows the state of secondary resonance (n=2).
The length of external auditory canal 8 is approximately 25 to 30 mm. That is, supposing that the sound speed at 15 degrees Celsius is 340 m/s and the length of the resonance box is 25 to 30 mm, a resonance frequency f1 of the primary resonance (n=1) shown in FIG. 1(b1) is found from the formula 2 as follows:f1=V/4L≅2833 to 3400 (Hz).A resonance frequency f2 of the secondary resonance (n=2) isf2=3V/4L≅8500 to 10200 (Hz).
A sound pressure-frequency characteristic obtained at the closed end, that is, at the eardrum position when the sound wave with a constant size is incident from an opening end of the resonance box by changing the frequency is shown by a graph in FIG. 2. Theoretically, since resonance occurs only at the resonance frequency, the sound pressure-frequency characteristic shows a sharp peak, but actually, the characteristic as distributed before and after that frequency is obtained.
Therefore, the sound pressure-frequency characteristics at the eardrum position are subjected to the influence of the one-end closed pipe resonance in the external auditory canal and have peaks at 2.8 to 3.4 kHz and at 8.5 to 10.2 kHz as illustrated in FIG. 2. That is, when the earphone is not attached, the eardrum hears sound in the outside world through an acoustic filter having the frequency characteristics illustrated in FIG. 2, and the reception sensitivity of the eardrum can be considered to have a frequency characteristic that the sound having the characteristics in FIG. 2 is heard flat when it is inputted. That is, it is the characteristics vertically reversed in the vertical axis direction in FIG. 2.
However, since the sound-isolating earphone 10 has the earplug structure having the ear pad 5, when the sound-isolating earphone 10 is attached as shown in FIG. 3(a), the earphone blocks the external auditory canal entrance 7 and changes the resonance mode. That is, the one-end closed pipe resonance changes to both-end closed pipe resonance with the both ends closed using the external auditory canal 8 as a resonance box.
FIG. 4 shows an internal structure of the sound-isolating earphone 10. As illustrated in FIG. 4, inside the earphone is constituted by an electro-acoustic transducer 2, a sound emitting port 15 which emits a sound wave to the external auditory canal entrance 7, and a sound leading portion 4 which connects the electro-acoustic transducer 2 and the sound emitting port 15. The electro-acoustic transducer 2 is protected by an external housing 1 and fixed to the external housing 1 by a suitable method, not shown.
The electro-acoustic transducer 2 is formed of a coil 21, a permanent magnet 22, and a diaphragm 23. The diaphragm is made of a thin plate of magnetic metal. By applying a current having an acoustic waveform to the coil, the diaphragm 23 vibrates in compliance with the acoustic waveform, and a sound wave is emitted toward the sound leading portion 4 in the direction to the right in FIG. 4. The rear face of the diaphragm 23, which is a sound emitting portion, is sealed.
As shown in FIG. 3, the sectional area of this sound emitting port 15 is smaller than the sectional area of the external auditory canal 8, and thus, reflection of the sound wave in the external auditory canal 8, which causes the standing wave, occurs on the end faces of the sound emitting port 15 and the ear pad 5 substantially without entering the sound leading portion 4. Therefore, the size, that is, the length in the depth direction of the external auditory canal 8 as the resonance box when the sound-isolating earphone is attached is determined by a position where the eardrum 9, the ear pad 5, and the sound emitting port 15 block the external auditory canal 8.
Actually, the position where the ear pad 5, and the sound emitting port 15 block the external auditory canal is slightly changed depending on the insertion condition of the earphone, but as shown in FIG. 3, it is assumed to be substantially equal to the position of the external auditory canal entrance 7, that is, it has the same pipe length as the case of the one-ended closed pipe. The actual length of the both-end closed pipe is also slightly different from the case of the one-end closed pipe, but the above assumption is made to facilitate the analysis.
FIG. 3(b1) and FIG. 3(b2) are explanatory diagrams of both-end closed pipe resonance and schematically illustrate the state in which the both-end closed pipe resonance occurs. A solid line indicates the both-end closed pipe and a broken line indicates amplitude of air vibration. In the both-end closed pipe resonance state in which the standing wave occurs, the amplitude of air at the positions of the ear drum 9, which is a pipe end, and the ear pad 5 inserted into the external auditory canal entrance 7 becomes the minimum (the pressure change is the maximum), and the air vibration at the position in the middle between the ear drum 9 and the ear pad 5 becomes the maximum (the pressure change is the minimum).
The resonance of the both-end closed pipe becomes the standing wave when the length of the pipe is the wavelength of n times as long as the half wavelength. Here, n is a positive integer. FIG. 3(b1) shows the case of the primary resonance (n=1), while FIG. 3(b2) shows the case of the secondary resonance (n=2).
As shown in FIG. 3(b1), if the pipe length of the both-end closed pipe is 25 to 30 mm, the standing wave having this length as the half wavelength becomes a resonance wave, and supposing that the sound speed at 15 degrees Celsius is 340 m/s, a resonance frequency f1′ of the primary resonance (n=1) is 5.7 to 6.8 kHz. Also, as shown in FIG. 3(b2), the secondary resonance (n=2) becomes the standing wave having the pipe length of 25 to 30 mm as 1 wavelength, and thus, a resonance frequency f2′ at that time is 11.3 to 13.6 kHz.
FIG. 5 shows the sound pressure-frequency characteristics at the eardrum position of the sound-isolating earphone. When the earphone is not attached, it becomes the resonance mode of the one-end closed pipe. The sound pressure-frequency characteristics assuming that the sound having a flat frequency characteristic equal to the sound source of the earphone is supplied to the external auditory canal entrance 7 is indicated by a broken line. When the earphone is attached, the characteristic becomes the resonance mode of the both-end closed pipe, and the sound pressure-frequency characteristic at the eardrum position in that case is indicated by a solid line. As shown in this figure, the sound pressure at the eardrum position when the earphone is not attached has peaks at 2.8 to 3.4 kHz and at 8.5 to 10.2 kHz, but the sound pressure peak at the eardrum position when the earphone is attached is subjected to the influence of the closed-pipe resonance in the external auditory canal and is displaced to 5.7 to 6.8 kHz and to 11.3 to 13.6 kHz, respectively.
The reception sensitivity characteristics of the human auditory system is such that the sound of any frequency is heard flat when sound with the frequency characteristics shown in FIG. 2 is inputted to the eardrum. As shown in FIG. 2, the sound around 3 kHz which is emphasized by resonance of the one-end closed pipe of the external auditory canal 8, and which constitutes a peak when the earphone is not attached changes to both-end closed pipe resonance mode when the sound-isolating earphone is attached, and does not constitute a peak around 3 kHz as indicated by a solid line in FIG. 5. Thus, the sound around 3 kHz is heard weaker than it actually is.
Also, since the sound around 6 kHz is emphasized by the both-end closed pipe resonance mode as indicated by the solid line in FIG. 5 when the sound-isolating earphone is attached, there is a problem that a quasi-sonant state occurs, and it sounds like an echo.
In order to solve this problem, as a general method, the frequency characteristic can be corrected by an electric method, but for that purpose, an amplifier and a filter circuit exclusive for the sound-isolating earphone need to be added, which complicates the circuit and requires a power supply. Reduction in size, weight and price cannot be realized easily with the earphone including such circuit. In order to realize reduction of size and price, a method of realizing a desired frequency characteristic only by an electric filter circuit can be considered, but if an amplifier is not included, lowering of the sound volume cannot be avoided.
In order to avoid difficulty of adding an electric circuit, some technologies to solve the problems unique to this sound-isolating earphone with a non-electric method have been proposed. As one of such examples, a technology of placing an acoustic resistor (damper) in a sound path and a technology of changing the length or an opening area of the sound path are disclosed (Patent Literature 1, Patent Literature 2).
According to the technology of Patent Literature 1, it is proposed that an acoustic resistor (damper) 6 is interchangeably installed in the middle of the sound path from an electro-acoustic transducer 2 inside the earphone to the sound emitting port 15 which leads the sound wave to the external auditory canal via the cylindrical sound leading portion 4 so as to adjust the sound quality of the earphone to preference of a user as means for suppressing high-frequency acoustics, which constitutes a problem.
FIG. 6 shows a sectional view of the earphone having the acoustic resistor 6. This is a general structure of an earphone having the acoustic resistor 6, and as the acoustic resistor 6, an unwoven cloth or a thin piece of foamed urethane is used.
FIG. 7 is a graph illustrating the sound pressure-frequency characteristics of the earphone having the acoustic resistor 6. A broken line indicates a characteristic when a sound-isolating earphone not having the acoustic resistor 6 is attached, while a solid line indicates a characteristic when the acoustic resistor 6 is provided for comparison. By referring to the sound pressure-frequency characteristic as the result of attachment of the sound resistor 6 as described above, it is understood that the peak around 6 kHz is suppressed.
Also, Patent Literature 2 proposes an adjustment pipe which can be detachably attached to the inside of an acoustic pipe installed on the side opposite to the sound-wave emitting direction and having different conditions with a different material or length and a method of providing a screw with different adjustment holes which can be interchanged for changing the opening area of the sound leading pipe or the acoustic pipe in order to change the frequency characteristics of the sound wave passing through the sound path.